Interpretive Summary: Neospora caninum is a recently discovered protozoan (single celled) parasite of livestock and companion animals. It causes abortion and neonatal mortality in cattle, sheep, goats, and horses. Its life cycle and sources of infections are unknown. Based on the life cycle of other closely related parasites, a carnivore host is suspected to excrete a resistant stage (oocysts) of Neospora in feces. Scientists at the Beltsville Agricultural Research Center and the University of California, Davis, California, tested carnivorous birds (vultures, crows, and owls) as a possible definitive host for Neospora. Although the results were negative the techniques described in this paper will be useful to other scientists who may wish to search for the definitive host for Neospora.

Technical Abstract:
Experimental oral infections to evaluate potential definitive hosts of Neospora caninum were conducted by feeding infected rodent tissues to 9 carnivorous birds of 4 species. Bird species included 2 red-tailed hawks (Buteo jamaicensis), 2 turkey vultures (Cathartes aura), 2 barn owls (Tyto alba), and 3 American crows (Corvus brachyrhynchus). The rodents (mice or rats) had been inoculated with 100,000 culture-derived tachyzoites of N. caninum 1 to 6 months before feeding to birds. Fecal samples were collected from each bird daily for 1 months after feeding rodents and examined for oocysts by fecal flotation. In addition, processed aliquots from all avian fecal samples were fed to BALB/c mice. Five weeks after feeding, mice were bled and sera were tested for antibodies against N. caninum. One to 2 months later, mice were killed and brain tissue was examined microscopically for protozoal cysts. While occasional oocysts were found in avian fecal samples, these were likely not N. caninum, because they were not infective to BALB/c mice. It was concluded that the bird species tested are not likely to be definitive hosts of N. caninum.